This invention relates to an eccentric, rotary plug valve used primarily for the modulating control of fluids in process plants. It is essentially an improvement over my similar invention patented under U.S. Pat. No. 3,623,696.
One of the requirements of such a valve is to obtain tight closure when a process system is shut down. The fact that eccentric valves have a somewhat complex geometry requires extremely accurate machining of distances between shaft center to plug seating surface, shaft center to orificial seating surface, length of contact area in the valve housing to seating area in orifice, and many others. To machine all these dimensions at very tight tolerances requires very expensive fixturing and tooling. Even if machining tolerances are held to the required level, there is also the problem of thermal expansion which, for example, is quite different for a stainless steel valve plug and a carbon steel body. All these problems have been recognized early on by experts in the field, and there are a number of patented devices that rely on various methods to solve this problem of insufficient geometrical alignment. In my U.S. Pat. No. 3,623,696 I utilize flexible arms between the hub and the plug portion of the rotary member. Various crank shaft mechanisms are shown in U.S. Pat. Nos. 3,575,376 or 1,233,856, and 3,675,894. Such solutions are not quite practical since they produce a degree of looseness between the moving parts and require a number of bearings which, while in the fluid stream, are easily contaminated or corroded. U.S. Pat. No. 4,817,916 shows a cam operated plug which again has similar drawbacks discussed previously. Other prior art is U.S. Pat. No. 3,963,211 showing a self-aligning valve plug utilizing spherical springs. This allows self-alignment in two planes but not in the horizontal direction between the shaft axis and valve seat. Also, no provisions are made to prevent the valve plug from "rattling" due to fluid turbulence. The hub or yoke is rounded for structural reasons only on very narrow sections, thereby negating any meaningful pressure recovery in the open position. U.S. Pat. No. 3,572,370 has a self-aligning plug for the horizontal direction but not for the other two planes, thereby limiting its usefulness. Additional requirements for o-ring seals severely limit the temperature range and negate applications for other than clean fluids. U.S. Pat. No. 4,074,889 shows mechanical adjustment means to control the distance between the center line of the shaft and the contact point between plug and orifice. However, this plug itself has no means for tilting and thereby will not be able to self-adjust in the other two planes. Again, yoke portions 19 are too narrow and too sharply rounded to allow for any meaningful pressure recovery as claimed in my inventions. Finally, U.S. Pat. No. 4,248,403 shows a two-part hub and plug arrangement in which a fixed extension of the hub supports the plug in a spring-loaded spherical joint. This allows for some degree of alignment. However, it does not guarantee that the center of the spherical seating surface is located on the center line of the seat ring in the correct distance relationship between plug and seat. This severely limits the sealing capability. Another problem of this invention is the fact that fluid forces tend to vibrate the plug around head portion 88 or 50. The biasing force of spring 66 is limited so as to allow rotation of plug 42 after the plug touches seat corner 76. With such a limited biasing force, there is no impediment to fluid turbulence trying to violently swing plug 42 back and forth while the valve is throttling in the open position.